1. Field of the Invention
This invention relates to a light switching element which uses an organic photoconductor, and a device, an apparatus, a recording apparatus, and a recording method which uses the light switching element.
2. Description of the Related Art
Recently, an optically writable space modulation device which is a combination of a photoconductive switching element and a functional element such as a display element has been developed, and has been practically used for a projector as a light valve, and the possibility of application in the optical information process has been studied as described in xe2x80x9cLiquid Crystal Space Modulatorxe2x80x9d, Liquid Crystal, Vol. 2, No. 1, 1998, pp 3-18.
In an optically writable space modulation device, the impedance of a photoconductive switching element is varied depending on the quantity of received light while a predetermined voltage is being applied on the element, the voltage applied on a display element is controlled to drive the display element, and an image is thereby displayed.
As the element for controlling the voltage or current by means of the quantity of received light, for example, the photo-diode used for CCD, a-Si used for contact type image sensors, and organic photoconductor have been researched and developed. Particularly, the organic photoconductor (OPC) is practically used for electrophotographic sensitive materials and solar cells because of small dark current, inexpensive material cost, and high productivity due to easy manufacturing, and also it is expected that the organic photoconductor is applied to a photoconductive switching element for the same reason.
OPC which has been proposed and used practically as an electrophotographic sensitive material has the structure as shown in FIG. 27 generally. An OPC layer having two layer structure of an charge generation layer 51 (CGL) and a charge transfer layer 52 (CTL) is provided on the surface of a conductive substrate 50. Ions are charged on the surface, and when a light is irradiated thereafter onto the surface, the ion charge disappears on the area where the light has been irradiated and on the other hand the ion charge remains on the area where the light has not been irradiated, that is the OPC functions as a light switching element. The mechanism which is involved in ion charge disappearance on the area where the light has been irradiated is described hereunder. When the charge generation layer 51 receives a light, the charge generation layer 51 generates carriers and electrons in the layer depending on the wavelength and the quantity of the light, and generates the charge. Generated electrons or carriers are transferred up to the surface through the charge transfer layer 52, and cancel the ion charge on the surface. As the result, the charge on the area where the light has been irradiated is discharged and the charge on the area where the light has not been irradiated remains. Afterwards, charged toner is adhered on the area where the charge remains, the toner is transferred onto a paper and fixed to obtain a printed image. The reason why the charge transfer layer is needed is that the withstand voltage of the charge generation layer is low. The charge transfer layer is necessary to improve the withstand voltage of the OPC used as a light switch.
The structure applied to a solar cell is, for example, shown in FIG. 28. In FIG. 28, a transparent substrate 60, a semi-transparent conductive film 61 made of a metal such as Al, an n-type inorganic photoconductive layer 62 made of a material such as SiO or ZnO, a p-type organic photoconductive layer 63 made of a material such as X-type non-metal phthalocyanine, and an electrode 64 are laminated in the order as described from the bottom. When a photon is incident onto a solar cell, a potential difference is caused at the interface between the n-type layer and the p-type layer, and the potential difference is utilized. In this case, the high withstand voltage is not necessary, and a charge transfer layer is not needed.
However, the organic photoconductive switching element is disadvantageous as described hereunder. In detail, the rectification function of the organic photoconductive switching element is a disadvantage. Usually, any one of electron and carrier is involved in transfer through a charge transfer layer. The reason is that an electron attractive material or an electron donative material is used as the charge transfer layer material. Though some materials such as polycarbazole can: be involved in bipolar transfer, the sensitivity is poor and such a material has not been used practically.
Because of this reason, the organic photoconductive cannot be applied to, for example, a image display apparatus in which a liquid crystal element is incorporated combinedly or an optically writable space modulation element. The reason in that the applied voltage is increased and the baking of an image is caused concomitantly. In detail, when a voltage is applied on OPC which has rectification function, the time during which an electric field is applied to a liquid crystal effectively is reduced because the positive and negative bipolar voltage application is difficult. Therefore, to control the orientation of the liquid crystal, it is required to apply a higher voltage or to apply a voltage for a longer time.
Because of the rectification function, the electric filled of one polarity cannot be applied and a DC bias is applied to the liquid crystal equivalently in the effect, ions in the liquid crystal are transferred to the place near the electrode because of the bias, the switching becomes difficult due to the electric field which ions generate, and the image baking occurs. To prevent the image baking due to transfer of ions in the liquid crystal, usually positive and negative alternate electric field is applied.
It is difficult to use not only the liquid crystal but also OPC having the conventional structure for switching of an element to which a DC bias component should not be applied effectively.
Because of this difficulty, a-Si is used as an photoconductive switching element for an light switching type image display apparatus having the liquid crystal as described in a literature xe2x80x9cReflective Display with photoconductive Layer and a Bistable, Reflective Cholesteric Mixturexe2x80x9d by H. Yoshida and T. Takizawa, SID 1996 APPLICATIONS DIGEST pp 59. a-Si is capable of bipolar charge generation and transfer, but a-Si is disadvantageous as described hereunder. The sophisticated film forming technique is required, and it is required to heat a substrate at a temperature higher than 200 degrees usually in the fabrication process, in which plasma CVD is employed usually, to result in difficult practical application to a plastic substrate, the production cost is high, and the process control is difficult, as the result a-Si is not practically used. Other photo-sensitive material such as Se or CdS may be likely alternative for a-Si, but these materials are very hazardous for the environment and human body.
The present invention was accomplished to solve the above-mentioned problem, and provides a high performance inexpensive photoconductive switching element used for light switching of a functional element driven by means of AC electric field or AC current, and also provides a device in which a photoconductive switching element and a functional element such as liquid crystal display element are combined, an apparatus, an recording apparatus, and a recording method in which the device is incorporated.
The photoconductive switching element switches a functional element driven by an AC electric field or AC current. The photoconductive switching element ha s at least a light transmissible electrode layer, a charge generation layer, a charge transfer layer, and charge generation layers. The layers are laminated in this order on a light transmissible substrate.
The photoconductive switching element of the present invention has two charge generation layers with interposition of a charge transfer layer, so the response signal with respect to the 0 V line obtained is close to symmetrical when an AC electric field is applied.
A photoconductive switching element in which charge generation material and charge transfer material are mixed at the interface between the charge generation layer and the charge transfer layer, and the mixing ratio changes continuously in the lamination direction of the layer is advantageous in performance. It is advantageous to use a plastic substrate, and organic material for forming the charge generation layer and charge transfer layer.
The present invention also provides a device in which the photoconductive switching element is connected electrically to a functional element the photoconductive switching element may be combined with the functional element in a piece.
If the functional element shows a memory characteristic, or is a display element, a display element showing a memory characteristic, a liquid crystal display element, a bistable liquid crystal display element, or cholesteric liquid crystal display element, the device of the present invention functions effectively.
The connection between the photoconductive switching element and a functional element is stabilized by combining these elements in a piece. Particularly, a device fabricated by combining the functional element showing the memory characteristic with a photoconductive switching element in a piece is detachable from the apparatus body which drives the device, and the separated device can be distributed.
The present invention also provides a device fabricated by successively laminating the photoconductive switching element, a functional film for removing the DC component, and a functional element such as a display element combined in a piece.
Because the functional film for removing the DC component is provided in the device of the present invention, the symmetry with respect to the above-mentioned 0 V line is further improved.
Also, the functional film for removing the DC component has a capacitance larger than that of the display element; therefore, the device functions effectively.
A device having a functional film for removing the DC component mainly made of an organic material selected from a group of polyvinylalcohol (PVA), polyvinylcarbazole, polyvinylacetate, polyethyleneoxide, and polybutylalcohol, or an inorganic material selected from a group of Sixe2x80x94O, Tixe2x80x94O, Alxe2x80x94O, Sixe2x80x94N, PZT, Taxe2x80x94O, and Alxe2x80x94N functions still more effectively.
The present invention also provides an apparatus including a device having the photoconductive switching element and any of a functional element showing a memory characteristic, a display element, display element showing a memory characteristic, a liquid crystal display element, a bistable liquid crystal display element, and cholesteric liquid crystal display element combined in a piece, and a driving mechanism connected electrically to the device, and this driving mechanism is detachable from the device.
Because the apparatus includes the device having the photoconductive switching element and the functional device and the driving mechanism which is detachable from the device, the device is detachable from the body for driving the device, and the separated device can be distributed.
The present invention provides an apparatus having the device fabricated by laminating the switching element, the functional film and the functional element, and the driving mechanism connected electrically to the device. The driving mechanism is detachable from the device.
The present invention also provides a recording apparatus including a device having the photoconductive switching element and the display element combined in a piece, a light writing unit for emitting a light onto the photoconductive switching element, and a pulse input unit for applying the positive pulse and the negative pulse on the device as the driving pulse for driving the device.
Because the photoconductive switching element is used in the recording apparatus, the high reflectance is obtained when an AC electric field is applied for driving.
The pulse input unit of the above-mentioned recording apparatus preferably applies a negative pulse as the final pulse, and a positive pulse as the first pulse, or applies a positive pulse as the first pulse and a negative pulse as the final pulse. In this context, the negative pulse means a pulse which is applied so that the voltage of the electrode on the display element side is higher than that of the electrode of the photoconductive switching element side of the device. On the other hand, the positive pulse means a pulse which is applied so that the voltage of the electrode of the photoconductive switching element side is higher than that of the electrode on the display element side of the device. The device is preferably provided with a functional film for removing the DC component additionally in this recording apparatus. Furthermore, in this recording apparatus, it is preferable that a cholesteric liquid crystal element is used as the display element, and a pulse input unit for applying final pulse of negative polarity is used as the pulse application unit (regardless of the polarity of the first pulse).
The present invention also provides a recording method in which the recording apparatus is used. In the recording method, the positive pulse and the negative pulse are applied to the device as the driving pulse for driving the device, a light is incident onto the photoconductive switching element for light writing, and a negative pulse is applied as the final pulse, a positive pulse is applied as the first pulse, or a positive pulse and a negative pulse are applied respectively as the first pulse and the final pulse.
In the recording apparatus and the recording method, because the negative pulse is applied as the final pulse (regardless of the polarity of the first pulse), the display element is turned on effectively in the case where a display element which requires a sharp voltage drop when a voltage is turned off such as a cholesteric liquid crystal display element is used.
Further, in the recording apparatus and the recording method, because the positive pulse is applied as the first pulse, an excellent modulation effect is obtained without adverse effect due to the time constant of a photoconductive switching element and a display element.